Hi, have been wondering the same, although I've been mixed feeding since day 1. DD is 5.5 weeks. I am finding that she is often not satisfied from the breast. Hospital said to stop the formula top-ups when the milk came in but I was engorged for ages and struggling to satisfy her even then so I carried on. I also have two older DC who like me to take them to school, pre-school, to parties etc so I often leave DD with DH or our nanny with formula for an hour or two (I'm planning to go back to work in 6 months and didn't want to lose our brilliant nanny, so I have help Mon-Thur, and often feel like I need to use the £ investment to spend time with my two young sons as well as DD).
I asked my HV about this and she said 'if it works for you, carry on.' DD is putting on the right amount of weight for her centile (25th) and is sleeping well at night (waking once or twice for a breast feed). Because of the other DC and because she seems to be 'less satisfied' at different times every day, there is no pattern to when we use the formula, but it totals to about 50-150mls a day. I was quite a 'purist' bf-er with my boys and would never have done this with them but it seems to be rubbing along alright. One thing I will say is that my boobs tend to fill up randomly, leak a lot more and ache/hurt a lot more (compared to last time), possibly because no two days are the same. If we didn't have our nanny and therefore I wasn't going off and doing so much stuff with the boys, I probably wouldn't be doing it but I feel like they need me as much as she does. And the fact is, she is often hungry after a feed still and has been since the start (altho I get that it's a vicious circle and would improve if I just feed through it for a few days and dropped the formula).
Finally, with my first I struggled to get him onto a bottle after months of EBF and with BTW coming relatively soon, at least this way I won't have any trouble getting her onto full time formula.
Hope that helps! Sorry, long ramble, but I guess this is proof it can be done.